Level Select
Press Y, Y, Y, X, X, B, X, Y, B, X, Y, X, Y, X, Y, B, Y, Y at the title screen, then pause the game to access the level select.
100% success rate. 

Credits
Press Y, X, B, Y, Y, X, B, Y, X, B, Y, X, X, B, Y, X, B, Y, X, B, B, Y, X, B at the title screen, then pause the game to access the cheat menu.

Invincibility
Press Y, X, X, B, B, B, Y, Y, X, X, X, B, B, B, B, X, Y, B, B, B, X, B, Y, B, B, X, B, B, Y, B, X, B, B, B, Y at the title screen, then pause the game to access the cheat menu.

Secret Level
Press Y, X, B, B, X, Y, Y, X, B, B, X, Y, Y, B, X, Y, X, B, B, X, Y at the title screen, then pause the game to access the cheat menu.

Go to: Finding Nemo Info Page | nGaming Home Page.